FUTURE SHAKES?
Oklahoma considered at ‘significant potential’ for damaging earthquakes
Much of northern and central Oklahoma face a “significant potential for damaging shaking” from man-made earthquakes despite a slowing rate of overall seismic activity in the state, according to a report from the U.S. Geological Survey on Wednesday.
The updated earthquake hazard map is the second annual map and report to include induced seismicity.
“The story is generally good news in terms of potential damage from earthquakes,” said Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project. “Generally the chance of damage has decreased. But the hazard risk in Oklahoma and southern Kansas still remains hundreds of times higher than before man-made activity began.”
The new map places the highest chance of a damaging earthquake at between 10 percent and 12 percent in an area centered near Pawnee, which on Sept. 3, 2016, experienced a magnitude 5.8 earthquake, the strongest recorded in state history.
“There is still work to be done. However, the total number fell compared to previous years” Peterson said. “The bad news is Oklahoma experienced the most magnitude 5.0 or greater than in recorded history.”
Oklahoma averaged about five magnitude 2.7 or greater quakes per day in 2015, but the rate fell to 3.6 per day last year and 1.4 per day so far this year, according to data
from the Oklahoma Geological Survey. Before 2008, the state averaged one or two such quakes a year.
The odds of large earthquakes generally decline as the number of smaller earthquakes drop, but not necessarily at the same pace, researchers said Wednesday.
“Unfortunately larger earthquakes don’t schedule themselves nicely,” said Jeremy Boak, director of the Oklahoma Geological Survey.
The largest earthquake associated with disposal wells at the Rocky Mountain Arsenal in Colorado occurred about a year and a half after injections stopped.
“The chance of a larger earthquake still exists even when wastewater injection is in decline and seismic activity is in decline,” said Daniel McNamara, a research geophysicist with the U.S. Geological Survey.
High-risk area shifts
While the 2017 map looks similar to last year’s, the area of highest risk shifted to an area between Pawnee and Cushing. Regulators have paid close attention to earthquakes around Cushing, which is home to the country’s largest commercial oil storage hub and a vast array of pipelines that distribute the oil throughout the country.
“We continue to monitor that area. Seismology is continuing to decline,” said Tim Baker, director of the Corporation Commission’s Oil and Gas Conservation Division. “If we see an anomaly where we suspect an increase in earthquake activity, we will take a look to see if anything needs to be done. But for now, everything is on a decline.”
Earthquake activity near Cushing is down about 75 percent from its peak, according to Oklahoma Geological Survey records.
Earthquake research and regulations have focused on saltwater disposal wells that take high volumes of water produced along with oil and pump them into the Arbuckle formation, which is the deepest sedimentary rock layer underlying much of Oklahoma.
Most wells in Oklahoma and throughout the world produce ancient saltwater along with oil and natural gas. The amount of water produced varies widely from formation to formation.
Oklahoma Corporation Commission regulations over the past two years have limited wastewater disposal at more than 700 disposal wells across more than 10,000 square miles in central and northwestern Oklahoma.
Boak said injection rates have fallen both because of market forces and because of regulations.
“What you can see through time is that the injection crested when oil prices started down and declined before the Corporation Commission began to put restrictions in place,” he said. “That could be attributed primarily to market forces. After that, it’s hard to tell whether it was a response to directives or simply a continued market-force driven change.”
As commodity prices have recovered and drilling activity has returned, regulations are limiting how much wastewater can be disposed of into the Arbuckle.
“We are seeing applications for zones other than the Arbuckle,” Baker said. “Operators are well aware it is going to be highly unlikely to dispose into the Arbuckle. They’re not looking to do that anyway. They’re looking in other areas.”
Companies are disposing about 1.5 million barrels of wastewater per day into the Arbuckle, down from more than 3 million barrels a day in 2014. A new Corporation Commission directive issued last week effectively caps the maximum potential disposal at about 2.6 million barrels a day.
Overall disposal also is dropping as the oil and natural gas industry has moved its focus away from the water-heavy Mississippi Lime formation and toward central Oklahoma’s STACK and SCOOP formations.
“The uptick is that the SCOOP and STACK have much less water,” Seismologist Jacob Walter said. “There’s also drilling going on in the Mississippi Lime, but it’s at a much slower pace. Operators are beginning new wells when volumes on their other wells are reduced to the point where they can stay within their allocated amount.”